Rotting organic matter in wetlands or beneath lakes can give rise to methane, which is a much more powerful greenhouse gas than the usual culprit, carbon dioxide.

Until recently most scientists believed that lakebeds were a minor source of the gas.

Methane bubbles have long been known to rise from the depths of Siberian lakes. But the bubbles were intermittent and difficult to measure.

Recently, though, an international team of U.S. and Russian scientists realized that the best time to look for bubbles was in winter. In the cold season researchers can walk across the frozen lakes, peering at bubbles trapped in the ice.

What they found is that lakebed emissions are large enough that taking them into account increases the total estimated level of Arctic emissions by 10 to 63 percent. The scientists report their findings in tomorrow's issue of the journal Nature.

Not So Permafrost

The methane appears to be produced from the thawing of permafrostwhat should be a permanently frozen layer of earthdue to global warming, says the study's lead author, Katey Walter of the University of Alaska Fairbanks's Institute of Arctic Biology.

Overall, she says, there is as much organic matter buried in the Siberian tundrarolling treeless plains with mucky soil and mosses at the surfaceas is contained in all of the world's tropical rain forests.